求助prep、 ps1 的第7题

7.    2311-!-item-!-187;#058&002635

In a certain board game, a stack of 48 cards, 8 of which represent shares of stock, are shuffled and then placed face down.  If the first 2 cards selected do not represent shares of stock, what is the probability that the third card selected will represent a share of stock?

(A)  1/8

(B)  1/6

(C)  1/5

(D)  3/23

(E)  4/23

正确答案是E，详解中没有解释

哪位大牛能教教我，我一点思路也没有啊？